Closing the Phosphorus Cycle on Vegetable Farms: Releasing Soil-Bound Phosphorus to Support Springtime Seedling Growth

Closing the Phosphorus Cycle on Vegetable Farms: Releasing Soil-Bound Phosphorus to Support Springtime Seedling Growth

Summary

Vegetables are often grown on high-phosphorus soils, but additional phosphorus is often required for early sowings. We have demonstrated that an new granular potassium bicarbonate can release phosphate quickly in such soils, and potentially replace the phosphorus addition. We are developing a protocol for using this material, determining on which vegetable soil types if is usable, as well as investigating high-phosphorus cover crop residue and seed-applied phosphorus as alternatives for supplying needed phosphorus until mineralization from soil is sufficient.

Objectives/Performance Targets

In high-phosphorus soils, identify which pools of P are available for release to spring-sown vegetable crops.

Test specific treatments that may make P available without further increasing soil P.

Accomplishments/Milestones

We tested P release on 31 soils representing the Northeast, using eight different measures of P availability. Some are highly correlated (i.e. Bray1, Bray2 and Mehlich3), but others measure different pools of soil P. None predicts the amount of P that is released by bicarbonate, so we don’t know which pool is being accessed. More sophisticated analysis of the data will be undertaken to further investigate likely relationships.

The P leaching potential (CaCl2 extractable P) was evaluated relative to that available for season-long growth (Morgan-extractable P). The leaching potential was substantially higher at every level of phosphorus that found in Delaware Co, NY dairy farms. (Kleinman, et al. Soil Science 165:943–950.)

As for testing specific treatments that may make P available without further increasing soil P, we will test several potential methods that have promise based on grower experience, knowledge of soil chemistry, and knowledge of rhizosphere biology.

We evaluated and eliminated citric acid and P-releasing microbes in 2000. In 2001 We emphasized bicarbonate and buckwheat cover crop.

Phosphorus response There are no data from replicated trials demonstrating that modern higher-yielding varieties respond to starter phosphorus in high-P soils. We tested two varieties, Hystyle and Zeus, at seven locations. The response, an average yield increase of 20%, was statistically significant in only two sites. However, all sites had a positive response of >10%. A 10% yield penalty will make eliminating starter P uneconomical, so a substitute treatment is needed.

Bicarbonate in furrow Bicarbonate as an alternative to starter P was tested at five sites. The rate selected was low (2 lb/ac), and the response was slight. We showed that a rate of 5 to 8 lb/ac will be safe, although rates over 10 lb/ac reduced stands. We propose to test the response to higher rates in 2002.

Phosphate coatings. We tested coatings of zinc phosphate and calcium phosphate at 10 mg/g seed, which is abut three times the amount present in the seed. There was no significant yield response at either site. Calcium phosphate showed a slight numerical increase in yield at both sites, a slight increase in tissue P, and a significant shift to smaller (more valuable ) beans at maturity.

Buckwheat cover crop. Buckwheat partitions phosphorus into the stem, and the stems decompose rapidly over winter. We tested whether this phosphorus would be available to bean plants in the spring. Strips of buckwheat and oats (control) were sown in a high-phosphorus field in late summer. Phosphorus in the cover crop and soil in each strip was measured when the cover crop froze in the fall, when the field was ready to till in the spring and at bean planting two weeks later. Growth of the beans was measured in the second year. There was no significant difference in rapidly-available (AEM) soil P among buckwheat, oat and bare-ground. The bean growth was unaffected by cover crop treatment. Thus there was no indication that buckwheat would provide the starter P needed by bean seedlings.

Impacts and Contributions/Outcomes

Vegetable soils in the Northeast are more susceptible to phosphate leaching than the field crop and dairy soils that have been studied previously. Therefore, vegetable growers in high-risk locations and high-P soils need to be aware of measures to reduce leaching.

Soils vary substantially in their ability to release phosphorus with small amounts of bicarbonate. The release is not predicted by the phosphorus measured by common soil test extractions.

Beans (Zeus and Hystyle) planted in mid May (the first 10 days of planting in the region) responded to banded starter phosphorus, averaging about 20% yield increase. The response was not predicted by the amount of phosphorus in the soil, nor by the content of phosphorus in leaves of young plants.

We have identified granules of potassium bicarbonate as a material that has low phytotoxicity, is easy to meter in common planting equipment and has have the ability to release substantial amounts of phosphate from test soils representative of the entire Northeast. Ongoing work will determine whether seedling tissue P and yield can be increased with bicarbonate; application rates used to date do not do so enough to substitute for banded P fertilizer.

Collaborators: